The use of carbon fiber for high temperature composite applications is of growing interest. Greater use of carbon fiber yarns is dependent, at least in part, on achieving improvements in handleability during yarn processing such as weaving and yarn delivery during take-off from the package, and in the production of composites with good resin dominated mechanical properties. The use of agents which might permit attainment of these objectives is limited to those which can withstand high temperatures encountered in production and use of parts from the composites. Thus, standard epoxy sizes, while providing yarns with excellent handleability and processibility, are unacceptable since they are unstable at the temperatures required in the cure cycles. Attempts to use these sizes would lead to degradation at the fiber-resin matrix interface where good adhesion is vital to obtaining the desired mechanical properties.
U.S. Pat. No. 4,923,752 to Cornelia provides a useful size for carbon fiber to be used in high temperature composites. The size is constituted by a fluorinated poly(amide-acid) in a mixture of defined proportions of an aprotic solvent and an alcohol. It is difficult to control the size concentration in an application bath because the solvent alcohol evaporates easily. As a result, the yarn size pickup is hard to control. Also, the size requires that certain precautionary measures be taken in handling both from the standpoint of safety and with regard to environmental considerations. The present invention provides a novel size composition that to a great extent obviates the need for such measures.